Acute Phase Proteins

The associations offibrinogen, serum amyloid A (SAA), and C-reactive protein (CRP) with CHD have been clearly established (4-6). Clinical assays for these three proteins are currently available. Fibrinogen is measured in clinical laboratories using more than 40 different methods employing functional assays. Although these methods are relatively precise (~10% reproducibility), they are not standardized. Therefore, significant variability in the measured fibrinogen in a particular sample is seen among laboratories. For example, according to the College of American Pathologists, fibrinogen values reported from the surveyed laboratories in a single sample varied from 121 to 437 mg/dL. Because patients' results are interpreted in the context of nationally established cut points, the use of standardized assays for risk assessment of heart disease is essential. A single assay for the determination of SAA is currently available but is not approved by the FDA for clinical use and is not accessible to most clinical laboratories in the United States (7). CRP is a highly stable protein that has been measured in clinical laboratories for several decades using standardized and FDA-approved methods. Historically, however, this protein has been used to measure active infection or inflammation. The assays used for that purpose are reliable but lack the desired level of sensitivity needed to assess CRP concentration in apparently healthy subjects to determine their future risk of CHD. Therefore, over the last decade high-sensitivity CRP (hsCRP) methods have been developed, and currently more than 30 such methods are available worldwide (8). These methods are capable of

Fig. 1. Physiological and pathological span of CRP concentration (mg/L) and limits of detection of traditional and high-sensitivity assays.

reliably measuring CRP at concentrations <0.3 mg/L (Fig. 1), which corresponds to about the 5th percentile of normal population distribution. For the reasons presented, the AHA/ CDC Laboratory Science Panel of the workshop on the use of markers of inflammation in cardiovascular disease concluded that of all the examined inflammatory biochemical markers, only CRP can make the transition from the research environment to routine clinical practice at the present time (9).

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